Thoracic aortas were isolated from 4-month-old, stroke-prone spontaneously hypertensive rats (SHRSP) and age-matched normotensive Wistar Kyotos (WKY) which had been maintained for 3 months on one of four experimental diets: standard (STD) (24% protein), low protein (LP) (19% protein), high protein (HP) (32% protein), or high methionine (Met) (1.9% methionine). The isometric tensions produced by low concentrations of norepinephrine (10-10, 10-9M) were similar in aortic strips from both SHRSP under various experimental diets (SHRSP/diet) and WKY under the standard diet (WKY/STD). The strips from SHRSP/STD and SHRSP/LP exhibited significantly reduced contractile responses to high doses of NE (10-8, 10-7M) and K+ (60 mM) while strips from SHRSP/Met and SHRSP/HP demonstrated contractile response values close to normal. Removal of Ca from the medium caused NE-induced contractions to decrease at a faster rate in strips from SHRSP/STD than in WKY/STD strips. The presence of LP in SHRSP strips accelerated the rate while Met and HP slowed the rate of decline of contraction. The contractile responses of all experimental strips to either NE or K+ demonstrated a biphasic configuration consisting of a fast and slow component. The values of both the fast and slow components in response to NE were significantly lower in strips form SHRSP/STD, SHRSP/LP and SHRSP/HP than in strips from WKY/STD and SHRSP/Met. Forskolin produced similar relaxation in strips from both SHRSP/diets and WKY/STD that were precontracted with NE. The results from these experiments indicate that (1) the differences in contractile response of aortic strips from normotensive WKY and SHRSP may be attributed essentially the the biochemical make-up of the membrane vesicles involved in Ca2+ transport and (2) in contrast to aortae from SHRSP/Met and SHRSP/HP, the aortae from SHRSP/STD and SHRSP/LP seem to have a much lower ability to retain and thus release appropriate amounts of Ca2+ required for muscle contraction.